Standby by DB couldn’t be brought as Read –Write till getting it Online first since it is in standby state for any further transaction log restore which is the main abstraction of transaction log shipping + DB Mirroring between a Primary (Online DB ) + a secondary DB (Standby by DB)

Therefore , you have to get the last Tail transaction log backup from the primary to capture all active transactions as well and then restore it on standby DB , then it should come back to read- write DB